feat: responsive compact mail view + folder unread badges

- CompactMailDelegate: card-based view for narrow panels (<420px)
  * Avatar with hash-based color, sender + date (relative format)
  * Subject line with attachment icon
  * Action buttons: flag, delete, category, more (context menu)
  * Unread highlight with light blue background + bold sender
- MailListView: automatic switch between tree/table (wide) and list/cards (narrow)
  * QStackedWidget with QTreeView + QListView sharing same proxy model
  * resizeEvent threshold at 420px
  * Signals forwarded to MainMainWindow for actions
- FolderTreeDelegate: custom paint for folder tree
  * Unread count badge (blue pill) right-aligned on folder nodes
  * Account nodes bold, folder nodes normal weight
  * Proper indentation per depth level
- MailItemDao::countUnreadByFolderId() for real-time unread counts
- MainMainWindow handlers for compact actions (flag, delete, category, more menu)

test_acc.cpp

@@ -0,0 +1,21 @@
+#include <QString>
+#include <QDebug>
+#include "src/core/models/account.h"
+#include <QCoreApplication>
+
+int main(int argc, char *argv[]) {
+    QCoreApplication app(argc, argv);
+    QString plain = "testPassword123";
+    QString enc = Account::encryptPassword(plain);
+    QString dec = Account::decryptPassword(enc);
+    qDebug() << "Plain:" << plain;
+    qDebug() << "Enc:" << enc;
+    qDebug() << "Dec:" << dec;
+    if (dec == plain) {
+        qDebug() << "SUCCESS: encrypt/decrypt roundtrip matches";
+        return 0;
+    } else {
+        qDebug() << "FAIL: mismatch";
+        return 1;
+    }
+}